Why is an intermediate layer placed between breakwater armor and its inner core?

In the design of a breakwater, an intermediate layer of smaller blocks or boulders is placed between the outer armoring and the inner core of rubble.1

This layer serves a critical structural purpose: it prevents the finer material that makes up the inner core from being dragged out by sea action through the gaps, or interstices, of the larger armor units. Without this protective layer, the loss of core material can lead to the settlement of the structure and potentially cause the breakwater to be breached by overtopping waves.1